Tissue-specific targeting of therapeutic proteins to tissues of choice in the body finds application in many medical conditions including cancer and a number of acquired and inherited disorders. For example, in the class of diseases called lysosomal storage disorders, an inherited deficiency in one or more enzymes which reside in the lysosomes leads to the accumulation of substrates for those enzymes in the cells. Because of tissue-specific patterns of expression and accumulation of the substrates within different cells in the body, these disorders result in tissue/organ-specific manifestations which vary depending upon the disorder. These disorders have been found to be treatable by intravenous administration of the active version of the enzyme deficient in the patient, a process termed enzyme replacement therapy (ERT). However, the efficacy of ERT varies widely among the different disorders. Although the reasons for this variability are not fully understood, it is commonly believed to be due to the rack of specific targeting to the most seriously affected tissues.
Most lysosomal proteins are glycoproteins containing one or more N- or O-linked oligosaccharide side chains of high mannose, complex or hybrid type. A number of receptors specific for these sugar residues exist, including among others, those for mannose, galactose (asialoglycoprotein receptor, ASGPR) and mannose-6-phosphate (cation-independent mannose-6-phosphate receptor, CIMPR). These receptors at least in part mediate the uptake of administered protein into cells. However, the distribution of these receptors within tissues in the body (e.g., ASGPR expressed on liver hepatocytes, mannose receptor on cells of the reticulo-endothelial system such as macrophages and Kupffer cells of the liver and CIMPR expressed widely on endothelial cells as well as other cell types) is not optimal for targeting proteins to the tissues which are most strongly affected. In some cases, modification and/or removal of a portion or all of the oligosaccharide chains through a process termed remodeling can advantageously improve the ultimate biodistribution of the proteins to more specifically target the protein to desired cell types (see, e.g., Furbush et al. Biochimica et Biophysica Acta 673:425-434 (1981), which describes sugar remodeling for a recombinant glucocerebrosidase, imiglucerase (Cerezyme®, Genzyme Corporation, Cambridge, Mass.)). However, complete removal of the carbohydrate side chains is often counterproductive, since they are also often necessary for the solubility and/or intracellular stability of the protein.
Another difficulty encountered with ERT is the strong immunogenicity of some therapeutic proteins as the patient's immune system often recognizes such proteins as foreign and mounts a neutralizing immune response. Thus, a means to reduce the exposure of the therapeutic proteins to the immune system would also be desirable.
Covalent conjugation with polymers such as polyethylene glycol (PEG) generally increases the serum half-life of a number of therapeutics such as antibodies, interferon, and effector molecules, while also reducing their immunogenicity. Although maintaining elevated concentrations of administered lysosomal proteins in circulation would similarly be expected to increase their bioavailability, in the case of lysosomal proteins, conjugation of these proteins with PEG (“PEGylation”) alone does not appear to be effective. This may partly be due to the adverse effect of the conditions in plasma, particularly elevated pH, on enzyme stability, and also on the inability of PEG, a neutral hydrophilic polymer, to influence the relative affinity of the glycoproteins for various receptor systems and to introduce any new tissue tropism to the protein. Thus, an additional means to promote uptake into the lysosomes of cells, and specifically the cells in those tissues in which substrate has accumulated in the body, would be highly desirable. In some cases, this can be achieved by the affinity of the polymer itself for specific tissue types (e.g., PVP-DMMan polymer conjugates for targeting a therapeutic to the kidneys are described in Kamada et al., Nat. Biotech. (2003) 21:399-404). Alternatively, it may be achieved by the introduction of ligands into the conjugate to promote interaction with tissue-specific receptors to mediate uptake. In the simplest case, such ligands are represented by antibodies against the receptor of choice. However, the larger proteinaceous ligands, such as antibodies, can themselves be immunogenic, thus posing significant challenges in the clinic.
Additionally, conjugation of a therapeutic protein with high molecular weight polymers may interfere with the activity of the protein at the site of action in the cell. For example, it has been found that many of the lysosomal enzymes, particularly those that act on glycolipid substrates, require a cofactor from the class termed saposins for their enzymatic activity. Saposins are believed to assist in presentation of the carbohydrate head group of the substrate to the catalytic site. Thus, conjugation of a high molecular weight polymer to the enzyme might affect the enzyme's activity by interfering with interactions with saposins, thereby lowering the efficacy of the therapeutic. Accordingly, a means to provide for elimination of the polymer from the enzyme in the site of action would be desirable.
On the other hand, another factor contributing to lowered efficacy of enzyme replacement therapies is the instability of lysosomal proteins within the lysosome, leading to a need for repeated administration. For example, Cerezyme® (glucocerebrosidase) is generally administered to a patient having Gaucher's disease on a biweekly basis due to loss of its activity after being taken up by target cells. The loss of activity is at least in part due to the action of lysosomal proteases on the protein, and appending polymers such as PEG can increase the resistance of proteins to proteolysis. Thus, under certain circumstances, a polymer may serve the additional function of protecting the protein in the lysosomal environment, thereby providing better intralysosomal stability of the active protein. Such a strategy may be effective in reducing the frequency of administration.
Low molecular weight ligands, such as peptides or mono- or oligosaccharides, may be used for targeting a therapeutic protein. However, such ligands often must be present in multiple copies on a macromolecule in order to mediate effective uptake by the cognate receptor, a condition termed “multivalent display.” Although current commercially available heterobifunctional PEGs (e.g., linear molecules containing different chemical entities on each terminus) may be used to generate ternary conjugates, they do not provide for multivalent display except by the attachment of multiple PEG molecules. But such heavy modification often has an adverse effect on enzyme activity.
Therefore, there exists a continuing need to provide protein therapeutics that allow for target-specific delivery within the body and are sufficiently biologically active upon intracellular uptake.